I haven't heard much more about this thing than what you've written, but I would guess it has more to do with them being over the ocean at the time burning off fuel than altitude. I am currently working on my private pilot's licence, and have used my cell phone at a few thousand feet (I assume they were higher than I usually fly though). It has worked just fine.
The cell phone companies don't like it because supposedly if you call from the ground, you are in contact with one or two towers, but from the air, you could be in contact with 6 or 8, which leaves less open connections for other callers.
EZ
The higher you are, the less likely a cell phone is to work. At 10,000 its basicly impossible. At 40,000 feet, it would require an Act of God himself.
There are two options when it comes to what happened with those cell phones on flight 93.
1) some one else made those calls from some where else.
2) God himself intervened.
At 10,000 feet, you're right, your cell phone probably won't work. That's all a good point about flight 93.
I stand by what I say about the Jet Blue flight though.
Several things conspire against cell phones working on an airplane.
1. You're in a (mostly) metal cabinet. Google for "Faraday cage" and you'll see that the airplane itself blocks a lot of the signal you're getting (and a lot of the signal you're giving).
2. Speed. Cell phone networks are tested to make sure that they can account for doppler effects at about 60 mph, not the 150-500 mph of jet airplanes. They might work, but you'd have to be lucky.
3. EZ noted that the flight was most likely circling over the ocean. While propagation over water is really good, it still only goes so far.
4. Cellular networks are designed with "down-tilt" in the antenna mounts. They point the antennas at the ground, not up in the air. This makes the signal above the ground weaker.
5. EZ also noted the visibility of multiple towers. This has several effects. First, in CDMA based networks, you can use multiple towers at the same time, and doing so from the air does reduce system capacity. TDMA/GSM systems speak to only one cell at a time. Talking to a cell with a radius designed for 2-3 miles (not uncommon in urban cores) while going 200 mph gives you, at best, just a few moments to come in to the cell, get it set up, speak, realize that you're about to hand off, find a candidate cell, and execute the handoff. You're crossing cell borders a lot faster than the system was designed to handle, and are more likely to lose the signal before the handoff can be completed.
What probably assisted flight 93 was that they were going low and slow, where cell service was better, over rural areas where the cells are much larger (perhaps a 10-15 mile radius). The JetBlue flight was over water, next to an urban area.
OK, braindump over. 
More than you ever wanted to know, eh?
"Faraday cage" means iron (i.e., steel), airplanes are aluminum, not applicable.
Talking to a cell with a radius designed for 2-3 miles (not uncommon in urban cores) while going 200 mph gives you, at best, just a few moments to come in to the cell, get it set up, speak, realize that you're about to hand off, find a candidate cell, and execute the handoff.
Not a problem, the handoff happens in less than a second.
Still suckin' on that blue pill aren't ya Bill?
Bill, Faraday cages are designed out of conductors, not a specific element.
Also, the handoff may take less than a second to execute, but from start to finish the process can take several seconds.
1. Call is already established.
2. Mobile and/or system determines that it is a "handoff candidate".
3. Mobile may need to announce this to the system (depending on technology).
4. System may need to take measurements of received signal strength in neighbor cells.
5. System may need to wait on these responses due to other phones needing handoffs as well.
6. System may need to make the decision on where to go. Is the target available?
7. System allocates resources in the target.
8. (Your count starts here) System informs mobile that it can hand off to target.
9. Mobile retunes/resynchs if necessary and begins communicating with target.
10. Mobile announces itself to target, and (depending on the technology) the previous serving resources are torn down.
For even a simple handoff, it can take a few seconds. Throw in lots of potential candidates, the mobile confusion of seeing more of the system than it should, and it can get longer than you'd like.
Why, yes, I am a cell phone geek! What made you think that?
man... ya meet the most bizarre people on blogs.
You kick ass IBCP.
Thank ya, sir.
Now I just need to buy that passel of water before Rita makes it this far inland.
something odd about goin to buy water when you're fixin' to get 2 feet of rain.
So much bird poop has gathered on my roof over this hot, dry summer that I don't want to take my chances with a cistern.
Besides, we're trying to be healthy and man cannot live on Diet Pepsi (which my wife likes, but I don't) alone.
IBCP, you're not thinking this through.
200 mph gives you, at best, just a few moments to come in to the cell, get it set up, ....
And at 200 mph it would take you well over a minute to go three miles, still plenty of time for the cell phone hand-off, even at a few seconds per hand-off, and hand-offs are transparent to the user. The only potential problem would be a rapid roll-off in signal strength due to interference, which is not as likely in a plane as it would be driving around a city in a car.
Bill, Faraday cages are designed out of conductors, not a specific element.
I've never seen one made out of AL, just steel, Mu-Metal, etc. Besides, it don't matter, a cell phone wavelength is on the order of, what, about 10cm, so there's a few hundred holes in an airplane that are well above the waveguide below cutoff size, particularly since the skin is only about 1/8" thick.
The only problem with using a cell phone in a plane is the directional antenna problem, and potential interference of having a bunch of 3 watt transmitters near the planes avionics.
200 mph can give you not that much time if you're traveling on the fringes of a cell. You'd have a minute if and only if you were directly traversing the cell. Being offshore and circling, I doubt they were.
As the airplane turned and the windows changed their orientation towards the various antennae, you'd have even more likelihood of rapid signal strength changes. Also, with signals bouncing around inside the airplane, the fading situation would be somewhere between horrendous and abominable.
The classic 800 MHz systems use wavelengths of about 36 cm. The newer 1900 MHz systems have wavelengths of about 15 cm. So yes, the windows would allow the signal in and out. However, this would be highly dependent on the orientation of the airplane. If the windows were at much of an angle compared to the antenna, you'd have a fight getting things in.
Thickness of the Faraday cage skin doesn't matter. In an RF shield room I ordered, the steel plating was not very thick, separated by a dielectric (wood), and then another plate.
Avionics and the increased RF exposure aren't the only problems with using cellphones in airplanes. Of course, a mini-switch could be designed that would use all major technologies and then reroute them to a satellite transceiver (PATENT NOTICE: I SAID IT FIRST!) on the outside of the airplane. The airlines could charge through the proverbial nose to have a good, solid cellular signal somewhere over the Pacific.
IBCP, the European airlines are experimenting with something like that now, albeit only over Western Europe, for now.
See here.
Also, mobile phones are only transmitting on the order of 1/2 a watt or less.
IBCP, you may also be interested in seeing these flood projections, depending on just how far inland you are.
Thickness of the Faraday cage skin doesn't matter.
The thickness of the material will have a bearing on the wave guide below cutoff number, not on the effectiveness of the shield.
Boy am I glad I don't live in Houston any more!!!
I will admit to not being an expert on Faraday cages and waveguides--I googled and found the "half wavelength" rule. Can you point me to some info regarding what you said about the thickness of the material?
And WaterBoy, I'm in D/FW, so it would take about a 500 foot plus storm surge traveling 200+ miles inland to get me with salt water, but what's gonna nail us is the rain.I live between Spring Creek and Duck Creek, so I _should_ be OK.
"I'm in D/FW..."
*grin*
A little out of range, eh? Carry on, then.
Can you point me to some info regarding what you said about the thickness of the material?
http://www.emcia.org/
Freeinforma...es_below_cutoff
The more I think of it, the more problems I see with thinking of a plane as a Faraday cage. For one thing, the aluminum is anodized, so it's not conductive between body sections, unless the rivets break the anodizing. Also, the doors and access panels don't have wire gaskets, so they'll act like huge gaping holes to RF. Keep in mind that if you have an opening that's 15 inches long, but only .002 inches wide, to an RF signal it might as well be a 15 inch square opening. A cell phone signal would have zero trouble penetrating a plane, the problems are all going to be with the ground antennas.
Regarding aluminum vs Mu Metal or steel for shielding; aluminum and copper are good for blocking higher frequency RF, but lousy for blocking any kind of magnetics, or even really low frequency radio waves, particularly higher power low frequency waves - the skin depth for current dissipation begins to be measured in inches instead of micro-inches (for high frequency stuff skin depth is generally measured in micro-inches). Aluminum and copper block magnetics about as well as air (seriously, air and aluminum both have relative permeability of 1, they're identical).
I'd guess the rivets put everything into electrical contact with each other. As they go in, I'd think they'd scratch the oxides off the outer plates and then the connecting bars of the frame. Mind, just a guess.
Now, when you do have that long, linear gap, it tends to polarize things, as I understand it. Wouldn't it also be directional, and you could only get a signal through if you were broadcasting straight onto it?
Yeah, Al stinks for magnetic shielding, but we need E shielding. Unfortunately, I don't fee like looking up permittivity constants to figure the skin depth at cellular wavelengths. I may be a geek, but I'm not sure I'm that much of one!
Commenting by HaloScan
